Method for making acoustic honeycomb

ABSTRACT

A method for making an acoustic structure that includes a honeycomb having cells in which septum caps are located. The septum caps are formed from sheets of flexible material that may be perforated before or after the material is inserted into the honeycomb. The flexible material is sufficiently flexible to allow folding into the shape of a septum cap. The flexible material is also sufficiently stiff to provide frictional engagement and locking of the septum cap to the honeycomb cell when the cap is inserted into the honeycomb during fabrication of the acoustic structure. An adhesive is applied to the septum caps after the caps have been inserted into the honeycomb cells to provide a permanent bond.

This application is a divisional of copending U.S. patent applicationSer. No. 13/279,484, which was filed on Oct. 24, 2011, which is acontinuation-in-part of U.S. patent application Ser. No. 12/961,112,which was filed on Dec. 6, 2010 and which has issued as U.S. Pat. No.8,066,098, which is a divisional of U.S. patent application Ser. No.12/151,886, which was filed on May 9, 2008 and which has issued as U.S.Pat. No. 7,854,298, which is a divisional of U.S. patent applicationSer. No. 11/099,337 which has issued as U.S. Pat. No. 7,434,659.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to acoustic systems that areused to attenuate noise. More particularly, the present inventioninvolves using honeycomb to make nacelles and other structures that areuseful in reducing the noise generated by a jet engine or other noisesource.

2. Description of Related Art

It is widely recognized that the best way of dealing with excess noisegenerated by a specific source is to treat the noise at the source. Thisis typically accomplished by adding acoustic damping structures(acoustic treatments) to the structure of the noise source. Oneparticularly problematic noise source is the jet engine used on mostpassenger aircraft. Acoustic treatments are typically incorporated inthe engine inlet, nacelle and exhaust structures. These acoustictreatments include acoustic resonators that contain relatively thinacoustic materials or grids that have millions of holes that createacoustic impedance to the sound energy generated by the engine. Thebasic problem that faces engineers is how to add these thin and flexibleacoustic materials into the structural elements of the jet engine andsurrounding nacelle to provide desired noise attenuation.

Honeycomb has been a popular material for use in aircraft and aerospacevehicles because it is relatively strong and lightweight. For acousticapplications, the goal has been to somehow incorporate the thin acousticmaterials into the honeycomb structure so that the honeycomb cells areclosed or covered. The closing of the cells with acoustic materialcreates the acoustic impedance upon which the resonator is based.

One approach to incorporating thin acoustic materials into honeycomb isreferred to as the sandwich design. In this approach, the thin acousticsheet is placed between two slices of honeycomb and bonded in place toform a single structure. This approach has advantages in that one canutilize sophisticated acoustic material designs that are woven, punchedor etched to exact dimensions and the bonding process is relativelysimple. However, a drawback of this design is that the strength of thestructure is limited by the bond between the two honeycomb slices andthe acoustic material. Also, the bonding surface between the twohoneycomb slices is limited to the surface area along the edges of thehoneycomb. In addition, there is a chance that some of the holes in theacoustic material may be closed with excess adhesive during the bondingprocess. It is important that the holes not be closed because this canresult in loss of active acoustical area of the resonator.

A second approach uses relatively thick solid inserts that areindividually bonded in place within the honeycomb cells. Once in place,the inserts are drilled or otherwise treated to form the holes that arenecessary for the inserts to function as an acoustic material. Thisapproach eliminates the need to bond two honeycomb slices together. Theresult is a strong structure in which the inserts are securely bonded.However, this approach also has a few drawbacks. For example, the costand complexity of having to drill millions of holes in the solid insertsis a major drawback, in addition, the relatively thick solid insertsmake the honeycomb stiff and difficult to form into non-planarstructures, such as nacelles for jet engines.

SUMMARY OF THE INVENTION

In accordance with the present invention, honeycomb acoustic structuresare provided in which individual sheets of acoustic material are formedinto septum caps that are inserted into the honeycomb cells. The septumcaps have a flange portion that is substantially thicker than theacoustic material and provide an anchoring surface that is used toattach the septum cap to the walls of the honeycomb. The septum caps areinitially held in place within the cells by frictional locking betweenthe anchoring surface and the cell walls. This frictional locking issufficient to keep the septum caps in position until they arepermanently bonded in place with an adhesive.

The acoustic structures of the present invention are designed to belocated near a source of noise, such as a jet engine or other powerplant. The structures include a honeycomb that has a first edge which isto be located nearest the source of noise and a second edge located awayfrom the source. The honeycomb includes a plurality of walls that extendbetween the first and second edge of the honeycomb. The walls of thehoneycomb define a plurality of cells wherein each of the cells has across-sectional area measured perpendicular to honeycomb walls and adepth defined by the distance between the first and second edges.

As a feature of the present invention, a septum cap is located within atleast one of the honeycomb cells and covers the entire cross-sectionalarea of the cell. The septum cap is made from a sheet of acousticmaterial that has a thickness and a perimeter. The sheet is preferablyrectangular in shape. The septum cap includes a resonator portion thathas an outer edge located adjacent to the honeycomb walls and a flangeportion that extends between the outer edge of the resonator portion andthe perimeter of the sheet of acoustic material. The flange portion hasan anchoring surface that is initially attached to the cell walls via africtional engagement to form a precursor structure. The anchoringsurface has a width wherein the width of the anchoring surface issubstantially greater than the thickness of the sheet of acousticmaterial so that it provides the required degree of frictional lockingbetween the septum caps and the honeycomb walls. The final acousticstructure is made by taking the precursor structure and applying anadhesive to the anchoring surface and the cell wall to permanently bondthe septum in place.

The present invention provides a number of advantages over existinghoneycomb acoustic structures. For example, there is no seam between twohoneycomb slices to weaken the structure. The septum caps may be placedat different levels within the honeycomb cells to provide fine-tuning ofnoise attenuation based on well-known Helmholtz resonator theory.Multiple septum caps may be placed in a single honeycomb cell atdifferent levels to create multiple cavities and impedance grids. Septumcaps made from different acoustic materials may be used in the samehoneycomb structure or even within the same honeycomb cell. The flangeportion provides a relatively large anchoring surface area to insuresecure bonding of the septum cap to the cell wall over the lifetime ofthe structure. In addition, the relatively thin and flexible septum capsdo not reduce the flexibility of the honeycomb, which is an importantconsideration for nacelles and other non-planar acoustic structure.

The above discussed and many other featured and attendant advantages ofthe present invention will become better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary acoustic structure inaccordance with the present invention.

FIG. 2 is a perspective view of an exemplary septum cap in accordancewith the present invention.

FIG. 3 is a cross sectional view of the exemplary septum cap shown inFIG. 2 taken in the 3-3 plane.

FIG. 4 is a cross sectional view of an exemplary acoustic structure inaccordance with the present invention where two sets of septum caps arelocated at two different depths within the honeycomb cells.

FIG. 5 is a cross sectional view of an exemplary acoustic structure inaccordance with the present invention where two septum caps are locatedwithin each honeycomb cell.

FIG. 6 is a schematic representation of a portion of the fabricationprocess for making acoustic structures where the septum cap is formedfrom a sheet of acoustic material and inserted into the honeycomb toform a precursor structure.

FIG. 7 is a sectional view showing an exemplary preferred method forapplying adhesive to the anchoring surface of the septum cap andhoneycomb wall by dipping the precursor structure into a pool ofadhesive such that the flange of the septum cap, but not the resonatorportion, comes in contact with the adhesive.

FIG. 8 is an exploded perspective view showing a portion of a solidskin, acoustic structure and perforated skin that are combined togetherto form an acoustic structure of the type shown in FIG. 9.

FIG. 9 is a partial sectional view of an exemplary acoustic structure(nacelle) that is located near a noise source (jet engine). The acousticstructure includes an acoustic honeycomb sandwiched between a solid skinand a perforated skin.

DETAILED DESCRIPTION OF THE INVENTION

An exemplary acoustic structure in accordance with the present inventionis shown generally at 10 in FIGS. 1 and 8. The acoustic structure 10includes a honeycomb 12 having a first edge 14 which is to be locatednearest the noise source and a second edge 16. The honeycomb 10 includeswalls 18 that extend between the two edges 14 and 16 to define aplurality of cells 20. Each of the cells 20 has a depth (also referredto as the core thickness) that is equal to the distance between the twoedges 14 and 16. Each cell 20 also has a cross-sectional area that ismeasured perpendicular to the cell walls 18. The honeycomb can be madefrom any of the conventional materials used in making honeycomb panelsincluding metals, ceramics, and composite materials.

As a feature of the present invention, septum caps 22 are located withinthe cells 20. It is preferred, but not necessary, that the septum caps22 be located in most, if not all, of the cells 20. In certainsituations, it may be desirable to insert the septum caps 22 in onlysome of the cells to produce a desired acoustic effect. Alternatively,it may be desirable to insert two or more septum caps into a singlecell.

An exemplary septum cap 22 is shown in FIGS. 2 and 3. The septum cap 22is formed from a sheet of acoustic material by folding the sheet into ahexagonal shaped cap that is sized to match the cross-sectional areas ofthe honeycomb cells. The septum cap 22 is preferably formed as shown inFIG. 6 by forcing the sheet 60 of acoustic material through a capfolding die 62 using plunger 63. The sheet 60 is preferably slightlyrectangular in shape and cut from a roll of acoustic material 64. Thesheet 60 has a thickness (t) as shown in FIG. 3 and a perimeter 65. Thesize and shape of the sheet 60 may be varied widely depending upon theshape/size of the honeycomb cell into which the sheet is inserted, thethickness of the sheet 60 and the particular acoustic material beingused.

Referring to FIGS. 2 and 3, the septum cap 22 includes a resonatorportion 24 that has outer edge 25. The septum cap 22 further includes aflange portion 26 that has an anchoring surface 27 which is initiallyattached to the cell walls 18 by friction engagement followed bypermanent bonding using an appropriate adhesive. The anchoring surface27 has a width (W).

The width (W) of the anchoring surface may be varied depending upon anumber of factors including the cross-sectional area of the cells, thethickness of the acoustic material, the type of acoustic material andthe adhesive. For a typical honeycomb having ¼ to 1 inch cells,anchoring surface widths on the order of 0.05 inch to 0.500 inch aresuitable for acoustic material that has a thickness on the order of0.001 inch to 0.10 inch. For standard acoustic materials having athickness of from 0.004 to 0.006 inch, anchoring surface widths of atleast 0.20 inch are preferred. In general, it is preferred that thewidth of the anchoring surface be substantially greater than thethickness of the acoustic material. “Substantially greater” means thatthe width of the anchoring surface is at least 5 times greater than thethickness of the acoustic material and preferably at least 20 timesgreater.

Any of the standard acoustic materials may be used to form the septumcaps. These acoustic materials are typically provided as relatively thinsheets that are perforated, porous or an open mesh fabric that isdesigned to provide noise attenuation. Perforated and porous sheets ofvarious materials (metals, ceramics, plastics) may be used. In onepreferred embodiment the acoustic material is an open mesh fabric thatis woven from monofilament fibers. The fibers may be composed of glass,carbon, ceramic or polymers. Monofilament polymer fibers made frompolyamide, polyester, polyethylene chlorotrifluorocthylene (ECTFE),ethylene tetrafluoroethylene (ETFE), polytetrafluoroethyloene (PTFE),polyphenylene sulfide (PPS), polyfluoroethylene propylene (FEP),polyether ether ketone (PEEK), polyamide 6 (Nylon, 6 PM) and polyamide12 (Nylon 12, PA12) are just a few examples. Open mesh fabric made fromPEEK is preferred for high temperature applications. Open mesh acousticfabrics and other acoustic materials that may be used to form the septumcaps in accordance with the present invention are available from a widevariety of commercial sources. For example, sheets of open mesh acousticfabric may be obtained from SEFAR America Inc, (Buffalo DivisionHeadquarters 111 Calumet Street Depew, N.Y. 14043) under the trade namesSITAR PFTEX, SEFAR NITEX and SEFAR PEEKTEX.

Another preferred embodiment involves the use of solid sheets ofacoustic material where perforations are formed in the material eitherbefore or after the septum cap is formed. Although metals, ceramics andplastics of the type identified above may be used for this embodiment,it is preferred that the acoustic material be PEEK or a similarchemically resistant polymer material that is suitable for hightemperature applications. Sheets or films of PEEK are availablecommercially from a number of sources, such as Victrex USA (Greenville,S.C.) which produces sheets of PEEK under the tradename VICTREX® PEEK™polymer.

PEEK is a crystalline thermoplastic that can be processed to form sheetsthat are either in the amorphous or crystalline phase. Films typicallyhave a thickness of from 0.001 to 0.006 inch. Compared to thecrystalline PEEK films, amorphous PEEK films are more transparent andeasier to thermoform. Crystalline PEEK films are formed by heatingamorphous PEEK films to temperatures above the glass transitiontemperature (T_(g)) of the amorphous PEEK for a sufficient time toachieve a degree of crystallinity on the order of 30% to 35%.Crystalline PEEK films have better chemical resistance and wearproperties than the amorphous films. The crystalline PEEK films are alsoless flexible and have more bounce-back than the amorphous film.Bounce-back is the force or bias that a folded film exerts towardsreturning to its original pre-folded (flat) shape.

Both crystalline and amorphous PEEK films may be used as septum capsprovided that one takes into account the difference in flexibility andbounce-back between the two materials when designing a particular septumcap for a particular honeycomb cell. In general, a thicker film ofamorphous PEEK is required to provide a septum cap that has the samedegree of friction-locking that is provided by a thinner crystallinefilm. For example, if a film of crystalline PEEK that is 0.002 inchthick is determined to have the required stiffness and bounce-back toprovide adequate friction-locking of a particular septum configuration,then one would need to use an amorphous film that is 0.003 inch thick ormore in order to achieve the same degree of friction-locking.

Solid films of PEEK or other plastic may be perforated using anytechnique that provides multiple openings in the solid film. Theperforations or holes may be drilled mechanically or using chemicals. Itis preferred that the perforations be made by laser drilling holesthrough the relatively thin PEEK film. In one embodiment, a flat sheetof PEEK film is laser drilled to provide the desired number ofperforations prior to forming the film into a septum cap. An advantageof this procedure is that the flat surface provided by the film makes iteasier to keep the laser beam focused on the film during the drillingoperation. In addition, the resonator portion and flange portions of theseptum cap are perforated without having to refocus the laser. Theseptum cap 22 shown in FIG. 3 includes perforations or holes 31 and 33in the resonator portion 24 and flange portion 26, respectively. Anadditional advantage of pre-drilling the entire film prior to folding itto form the septum is that the holes 33 in the flange portion 24 provideadded surface area and openings where adhesive can enter to improve thebonding of the flange to the cell wall.

In another embodiment, the resonator portion of the septum cap is notlaser drilled until after the septum cap has been inserted into thehoneycomb. As shown in FIG. 6, a solid sheet of PEEK 60 is formed into aseptum cap 22NP (Not Perforated) and inserted into honeycomb 12P. Theresonator portion of the septum cap is then perforated by laser drillingto provide holes 35. The laser drilling of the resonator portion may beconducted before or after the septums are permanently bonded into thehoneycomb. It is preferred that the laser drilling be delayed untilafter the septums have been permanently bonded in place. An advantage ofthis procedure is that in some situations, especially where numerousperforations in the film are necessary, one can use a thinner film thanis possible when the septum film is pre-drilled. The inclusion ofnumerous holes in the film tends to reduce the stiffness and level ofbounce-back of the film so that the resultant friction-locking of theseptum within the cell is also reduced. Accordingly, by delayingdrilling of holes until after the septum has been inserted,friction-locked and permanently bonded within the cell, one can obtainthe maximum bounce-back and friction-locking that is possible with agiven film. In addition, laser drilling the holes after the septums arein place avoids the possibility of perforations being inadvertentlyblocked by misplaced adhesive. A disadvantage of this embodiment is thatthe resonator portion of the folded septum may not be entirely flat sothat the laser may need to be refocused during the drilling operation.

The septum caps 22 may be inserted into the honeycomb cell to provide awide variety of acoustic designs. For example, the septum caps may belocated at different levels within the honeycomb 12 a as shown at 22 aand 22 b in FIG. 4. This type of design allows fine-tuning of the noiseattenuation properties of the acoustic structure. The two-level designshown in FIG. 4 is intended only as an example of the wide variety ofpossible multi-level septum arrangements that are possible in accordancewith the present invention. As will be appreciated by those skilled inthe art, the number of different possible septum placement levels isextremely large and can be tailored to meet specific noise attenuationrequirements.

Another example of an insertion configuration for the septum caps 22 isshown in FIG. 5. In this configuration, two sets of septum caps 22 c and22 d are inserted into the honeycomb 12 b to provide each cell with twoseptum caps. As is apparent, numerous possible additional configurationsare possible where three or more septum caps are inserted into a givencell. In addition, the multi-level insertion design exemplified in FIG.4 may be combined with the multiple insertion per cell designexemplified in FIG. 5 to provide an unlimited number of possible septumcap insertion configurations that can be used to fine tune the acousticstructure to provide optimum noise attenuation for a given source ofnoise.

As previously mentioned, the preferred method for inserting the septumcaps into the honeycomb is shown in FIG. 6 where the septum cap ispre-formed using cap folding die 62 and plunger 63. The referencenumerals used to identify the honeycomb structure in FIG. 6 are the sameas in FIG. 1, except that they include a “p” to indicate that thestructure is a precursor structure wherein the septum caps are not yetpermanently bonded to the cell walls.

It should be noted that the use of a cap-folding die 62 to form theseptum cap from the individual sheets of acoustic material is preferred,but not required. It is possible to use the honeycomb as the die andform the septum cap by simply forcing the sheet 60 into the cells usingplunger 63. However, the edges of many honeycomb panels tend to berelatively jagged because the panels are typically cut from a largerblock of honeycomb during the fabrication process. Accordingly, thehoneycomb edges tend to catch, tear and contaminate the acousticmaterial when a flat sheet of material is forcibly inserted directlyinto the cell. Accordingly, if desired, the cap-folding die may beeliminated, but only if the edges of the honeycomb are treated to removeany rough or jagged edges.

It is important that the size/shape of the sheet of acoustic materialand the size/shape of the die/plunger (or just the plunger if the die isnot used) be chosen such that the septum cap can be inserted into thecell without damaging the acoustic material while at the same timeproviding enough frictional contact between the anchoring surface andthe cell wall to hold the septum cap in place during subsequent handlingof the precursor structure. Routine experimentation can be used todetermine the various sizes and shapes of acoustic sheets that arerequired in order to achieve the necessary frictional locking or holdingof the septum caps in place in the precursor structure prior topermanent bonding of the anchoring surface to the cell wall withadhesive. The amount of frictional locking or holding should besufficient to keep the septum caps from falling out of the honeycomb,even if the precursor structure is inadvertently dropped duringhandling.

For a standard ⅜ inch composite honeycomb made from conventionalmaterials, such as fiberglass, phenolic, Nomex and aluminum, the sheetof PEEK film material (0.001 to 0.015 inch thick) can be a rectangle 65or a shape 67 matching the cell shape as shown in FIG. 6. Forrectangular sheet, the rectangle should have dimensions ranging from0.50 to 0.70 inch by 0.60 to 0.80 inch. For film material that is cut tomatch the shape of the cell, the sheet should be oversized a sufficientamount to provide a septum cap having the desired flange portion width.With respect to rectangular sheets that are folded into a septum cap, itis preferred that the sheet of acoustic material is not notched orotherwise cut in an effort to enhance folding of the sheet. It was foundthat the sheets, without notching, folded into septum caps that hadwrinkles in the anchoring surfaces that enhanced the bonding of theseptum caps to the honeycomb walls. In addition, notching tends torelease some of the outward tension or bias force (bounce-back) thatwould otherwise be present in the flange portion of the septum cap. Thisoutward bounce-back force or bias is a result of the polymer in thefolded sheet being inherently biased back towards an unfolded position.This outward force or bounce-back is an important part of the frictionallocking between the septum cap and the cell wall.

Frictional-locking of the septum cap is achieved by using a combinationof flange size, film stiffness/bounce-back and packing of septummaterial in the corners of the honeycomb. Hexagonal septum caps that areformed from rectangular sheets of material 65 tend to have extramaterial that can be compressed into the corners of the cell to provideadditional friction-locking when relatively thin films with low-bounceback are used. In order to reduce weight and wrinkling of the film, itis preferred that the sheets of film used to form the septum cap have aperimeter (67 in FIG. 5) that more closely resembles the final septumcap shape, so as to form a more uniform flange. In this preferredconfiguration, the size of the flange and bounce-back of the filmprovide substantially all of the friction locking of the septum cap tothe cell wall. For this type of preferred septum cap configuration,materials that are more flexible and have less bounce-back generallyrequire larger flanges than materials that are less flexible and havemore bounce-back.

The degree of frictional locking of the septum to the honeycomb can bemeasured by placing test weights onto the septums and determining ifthere is any resulting movement of the septum. For example, a septum isconsidered to be frictionally locked to the honeycomb wall with anacceptable amount of locking force if it passes the following test. A 27gram test weight is placed on top of the dry septum from the insertedside. The friction locking force is acceptable when the dry cap willsupport the 27 grams without sliding down the honeycomb cell. In anexemplary test, the 27 gram test weight is a steel rod that is 0.368inch in diameter and 2.00 inches long.

With respect to films of PEEK (thicknesses of 0.001 to 0.015 inch), thefilms all are sufficiently flexible to be formed into septum caps.However, the particular film that is used for a particular size andshape of septum cap will be determined by changing the film bounce-backby varying film thickness and film type (crystalline or amorphous) aswell as varying the flange width to establish the combination of filmbounce-back and flange width that is necessary to friction-lock theseptum to the cell walls.

The number and size of holes that are drilled in the septum cap, as wellas the hole pattern, may be varied depending upon the desired finalacoustic properties for the acoustic structure. The holes orperforations will typically vary in size from 0.002 to 0.015 inch andare preferably circular in shape. Holes that are not circular may beused, if desired. Other suitable hole shapes include elliptical, squareor slotted. The number of holes drilled in the resonator portion willvary depending upon hole size and desired acoustic properties. For holesthat are from 0.002 to 0.015 inch in diameter, it is preferred that thenumber of holes range from 100 to 700 per square inch for most acousticapplications. It is preferred that the number of holes and hole size beselected to provide the Rayl value and the Non Linear Factor (NLF)required for the individual acoustic application. The NLF will increaseas fewer larger holes are used to meet the Rayl requirements, while alower NLF will be produced by increasing the number of smaller holes tomeet a similar Rayl requirement.

The surface area of the holes should be kept below 20 percent of theoverall resonator portion surface area in order to maintain filmintegrity and to provide sufficient bounce-back for friction-locking tothe cell walls. If desired the number and size of the holes may bevaried between the resonator portion and the flange portion. This allowsthe use of one hole configuration in the resonator portion to maximizeacoustic properties while also allowing the use of another holeconfiguration in the flange to maximize adhesive interaction andresultant bonding to the cell wall.

A precursor structure is shown at 10p in FIG. 6 where the septum caps 22are held in place only by frictional locking. As mentioned previously,the frictional locking must be sufficient to hold the septum capssecurely in position until they can be permanently bonded using anappropriate adhesive. The adhesive that is used can be any of theconventional adhesives that are used in honeycomb panel fabrication.Preferred adhesives include those that are stable at high temperature(300-400° F.). Exemplary adhesives include epoxies, acrylics, phenolics,cyanoacrylates, BMI's, polyamide-imides, and polyimides.

The adhesive may be applied to the anchoring surface/cell wall interfaceusing a variety of known adhesive application procedures. An importantconsideration is that the adhesive should be applied selectively only tothe flange anchoring surface/cell wall interface and not to theresonator portion of the septum cap. Application of the adhesive to theresonator portion will result in closing or at least reducing the sizeof the openings in the mesh or other acoustic material. Any adhesiveapplication procedure that can provide selective application of adhesiveto the anchoring surface/cell wall interface may be used.

An exemplary adhesive application procedure is shown in FIG. 7. In thisexemplary procedure, the honeycomb 12 p is simply dipped into a pool 70of adhesive so that only the flange portions of the septum caps 22 p areimmersed in the adhesive. It was found that the adhesive could beaccurately applied to the anchoring surface/cell wall interface usingthis dipping procedure provided that the septum caps were accuratelyplaced at the same level prior to dipping. For septum caps located atdifferent levels, multiple dipping steps are required. Alternatively,the adhesive could be applied using a brush or other site-specificapplication technique. Some of these techniques may be used to coat thecore walls with the adhesive before the septum cap is inserted.Alternatively, the adhesive may be screen printed onto the septummaterial and staged before insertion into the core

The dipping procedure for applying the adhesive as depicted in FIG. 7was found to work particularly well because any wrinkles present in thefolded sheets of acoustic material provide small channels between theanchoring surface and cell wall that allows adhesive to be more easilywicked upward by capillary action. This upward wicking provides forfillet formation at the intersection of the outer edge of the resonatorportion and the cell wall. The formation of adhesive fillets at the edgeof the resonator portion not only provides for good bonding to the cellwall, but also provides a well-defined boundary between the adhesive andthe resonator portion to insure that the acoustic properties of theresonator portion are not adversely affected by the adhesive.

The acoustic structures in accordance with the present invention may beused in a wide variety of situations where noise attenuation isrequired. The structures are well suited for use in connection withpower plant systems where noise attenuation is usually an issue.Honeycomb is a relatively lightweight material. Accordingly, theacoustic structures of the present invention are particularly wellsuited for use in aircraft systems. Exemplary uses include nacelles forjet engines, cowlings for large turbine or reciprocating engines andrelated acoustic structures.

The basic acoustic structure of the present invention is typicallyheat-formed into the final shape of the engine nacelle and then theskins or sheets of outer material are bonded to the outside edges of theformed acoustic structure with an adhesive layer(s). This completedsandwich is cured in a holding tool, which maintains the complex shapeof the nacelle during the bonding. For example, as shown in FIG. 8, theacoustic structure 10 is heat-formed into the final nacelle shape. Thesandwich part is made by placing the solid sheet or skin 80 into thebonding tool. Next, a layer of adhesive is placed on the skin. This isfollowed by the addition of the shaped acoustic structure 10. The secondlayer of adhesive film is added and then the top skin 82. This completesthe sandwich. The assembly is bonded with heat and pressure. The finalnacelle shape is controlled by the bond tool. The panel will thenconform around the jet engine, which is shown diagrammatically at 90 inFIG. 9. Examples of Practice are as follows:

EXAMPLE 1

The following example provides details regarding an exemplary acousticseptum cap honeycomb in accordance with the present invention. It willbe recognized by those skilled in the art that a wide variety ofdimensions, honeycomb material, acoustic mesh material and adhesives maybe used. In general, the particular structural and acoustic applicationwill determine the various design requirements, which include coredensity, septum depth, acoustic impedance, core thickness, slice length,slice width and mesh material dimensions.

Exemplary Septum Core Product:

-   An exemplary acoustic septum cap core was made from fiberglass    honeycomb with ⅜-inch cells. The septums were located 0.500 inch    from the edge of the core, which was 1.25 inch thick. The acoustic    impedance of the septum core was found to be 70 rayls.

Materials:

-   Honeycomb was supplied by Hexcel Corporation (Dublin, Calif.) and    identified as Part number—HRP-⅜-4.5 pounds per cubic foot (pcf)    (0/90 degree fiberglass architecture with phenolic resin). The    density of the honeycomb was 4.5 pounds per cubic foot. Acoustic    Mesh was obtained from SEFAR America, Inc. which was identified as    Part number—17-2005-W022 (Nonmetallic woven mesh with an acoustic    impedance range from 45 to 64 rayls).-   The adhesive was obtained from Hexcel Corporation and identified as    Part number—899-55. The adhesive is in the Polyamide-imide family,    which is a proprietary material. Other adhesives, such as epoxies,    acrylics, phenolics, cyanoacrylates and polyamides, may be used, if    desired.

The Acoustic Core Dimensions were as Follows:

-   Core cell size: Typical cell size was 0.396 inch hexagonal inside    dimensions measured from wall to wall. Slice thickness was typically    1.250 inch. The mesh inserted into the hexagonal cells was typically    0.700 inch by 0.650 inch rectangular shape. The mesh was folded to    form the cap and inserted into honeycomb cell. The top of the cap    conforms to the cell shape and size (hexagonal shape with inside    dimensions of 0.396 inches). The side of the cap conforms to the    honeycomb cell wall for adhesive attachment. The sides of the cap    are typically 0.1875 inch long and are dipped into the adhesive for    attachment of the septum cap to the honeycomb.

Adhesive Dipping and Curing Process:

The Honeycomb Core with the Septum Caps Inserted into each Cell isDipped as Follows:

-   -   a. The core is placed into a tank of adhesive with the top of        the septum in the up position.    -   b. The slice is lowered to a set level, which allows the        adhesive to move up the honeycomb slice thickness and cover the        bottom sides of the cap.    -   c. The adhesive clip level up the side of the cap is typically        0.150 inch. The adhesive will wick up the last typical 0.0375        inch to close and lock the mesh fibers and bond the cap to the        honeycomb wall.

The Adhesive Cure Cycle is Accomplished as Follows:

Immediately after dipping and draining, the core is placed into a 300°F. oven. The adhesive is subjected to a cure cycle of 300° F. for 30minutes, 350° F. for 30 minutes and 400° F. for 30 minutes.

Acoustic Testing of Mesh and Septum Core:

-   -   1. The above meshes provided by SEFAR America, Inc. can be        adjusted by the supplier provide a range of acoustic impedances        from 25 to 120 rayls.    -   2. The acoustic impedance range for the septum core can also be        adjusted by the amount of adhesive placed on the mesh. Using an        example of 50 rayl mesh that is inserted into the honeycomb. If        the adhesive dip level is 0.100 inch up the sides of the cap.        The additional unsealed mesh above the adhesive line will reduce        the final core impedance in the cell to a typical 42 rayls. This        would be the lowest impedance available with this design. If the        adhesive seals up to the 0.1875 inch level—the typical impedance        will be 70 rayl.

Test Methods for Mesh and Core:

-   Two methods of testing can be used for acoustic evaluation. The    Raylometer or an individual cell vacuum testing for air    permeability. The raylometer units are in rayls and the individual    cell vacuum units are in K Pascals. The following table sets forth    the results of an acoustic evaluation of acoustic septum cap    honeycombs where the caps were mesh only (no adhesive) and where the    caps were bonded into place with adhesive, as described above.

Raylometer Vacuum Method Method 17-2005-W022 (Mesh Only) 50 Rayls 32 KPascals Septum Core (with adhesive) 70 Rayls 31 K PascalsThe vacuum reading for the mesh only core was made using a 0.250 IDvacuum test head with the mesh sealed against the opening. The vacuumreading for the Septum Core was made inside one 3/8-inch septum cell.This is similar to a 0.396 inch ID test head. The vacuum head wascalibrated as follows: Vacuum reading when open to the atmosphere 20 KPa and when completely sealed to atmosphere 80 K Pa.

It should be noted that the acoustic impedance readings decrease as thearea of mesh (more holes) increases. The typical resonator mesh has anopen area of 2% to 4%. When sound waves pass through the acoustic mesh,the pressure of the waves causes the particles of the mesh to move. Thesound impedance is the ratio of pressure and the particle velocity itproduces in the mesh. In other words: The acoustic impedance is thepressure of the sound waves on the mesh divided by the instantaneousparticle velocity of the mesh. As mentioned above, the unit of measurehere for acoustic impedance is the rayl. The actual rayl units are in“pascal-seconds per meter”. The acoustic impedance and vacuum pressuredrop across the mesh material is a. function of the open area (numberand size of holes per unit area).

For example: when using Sefar mesh part number 17-2005-W022, thepressure drop for different sizes of circular mesh areas in septum cores(prepared as described above) were as follows:

Mesh Diameter Mesh Area Vacuum Pressure Drop Inches Sq-Inches K-Pascals.355 .099 31 .375 .110 29 .510 .204 26 .570 .255 23This table shows that the number of holes increases with mesh area andthe pressure drop across the larger septum mesh area is lower.

The Sefar mesh part number 17-2005-W022 used in the exemplary septumcore, as described above, had a 0.355 inch diameter opening in theseptum cap mesh, which gave vacuum readings of 31 K-Pascals and Raylreadings of 70 Rayls for this design.

When the vacuum drop is measured across the acoustic mesh in the ⅜ inchhoneycomb cells the reading can range from 25 to 35 K-pascals and theacoustic impedance of the mesh in the ⅜ honeycomb cell will range from50 rayls to 120 rayls.

As is apparent from the above example, the use of differing amounts ofadhesive to bond the septum caps to the honeycomb provides one with theability to increase or decrease the effective amount of area of mesh inthe hexagon cell. This allows one to control the acoustic rayl value.For Example: If 60 rayl mesh is used in the septum cap. The cellimpedance can be lowered to 50 rayls by allowing the mesh around the topsides of the cap to not be covered with adhesive. This approachgenerates more open area of mesh in the cell and will lower theeffective acoustic impedance. If the adhesive is completely covering thesides and part of the radius between the vertical sides of the cap andthe horizontal top of the septum cap the impedance will increase to 75rayls.

EXAMPLE 2

Acoustic structures are made in the same manner as Example 1, exceptthat solid films of crystalline VICTREX®PEEK™ are substituted in placeof the PEEK mesh material. The solid films have thicknesses of between0.001 and 0.015 inch. The films are cut so as to form hexagonal sheetsthat are from 0.1 to 0.4 inch larger than the 0.396 inch insidedimension of the hexagonal cell. This provides septum caps having flangeportions that range in width from about 0.1 inch to 0.4 inch. Thehexagonal sheets of PEEK are formed into septum caps having a resonatorportion that matches the 0.396 inch inside dimension of the hexagonalcell. The various septum caps are inserted into the honeycomb. Adequatefriction-locking of the septum to the honeycomb is then tested by usingthe test method that was mentioned previously where a 27 gram weight isplaced on the septum. Those septums that pass the test and remainfriction-locked in the honeycomb are suitable for mass fabrication andinsertion into honeycomb for final adhesive bonding as set forth inExample 1. After adhesive bonding, the resonator portions of the septumsare laser drilled to provide a perforated septum cap having from 100 to700 holes per square inch where the holes range in size from 0.002 to0.015 inch in diameter.

EXAMPLE 3

Acoustic structures are made in the same manner as Example 2, exceptthat the solid films of crystalline VICTREX®PEEK™ are laser drilledprior to folding into septum caps to provide perforated films havingfrom 100 to 700 holes per square inch where the holes range in size from0.002 to 0.015 inch diameter.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodification may be made with the scope of the present invention.Accordingly, the present invention is not limited to the above preferredembodiments and examples, but is only limited by the following claims.

What is claimed is:
 1. A method for making an acoustic structure, saidmethod comprising the steps of: providing a honeycomb comprising a firstedge and a second edge, said honeycomb further comprising a plurality ofwalls that extend between said first and second edges, said wallsdefining a plurality of cells wherein each of said cells has across-sectional area measured perpendicular to said walls and a depthdefined by the distance between said first and second edges; providingat least one sheet of flexible material that has a thickness and aperimeter, said flexible material being sufficiently flexible to befolded into the shape of a septum cap for insertion into said cell, saidseptum cap having a resonator portion that extends transversely acrosssaid cells and which has an outer edge to be located at said walls and aflange portion that is located between said outer edge of said resonatorportion and the perimeter of said sheet of acoustic material, saidflange portion extending parallel to said walls and comprising ananchoring surface having a width; forming said sheet of flexiblematerial into said septum cap; locating said septum cap into said cellsuch that said septum cap is held in place only by friction locking ofsaid septum cap to said walls, said anchoring surface being sufficientlywide and the bounce-back of said flexible material being sufficient toprovide friction-locking of the septum cap to said cell walls; andadhesively bonding said anchoring surface to the wall of said honeycombto form said acoustic structure.
 2. A method for making an acousticstructure according to claim 1 wherein said sheet of flexible materialis perforated prior to forming said sheet of flexible material into saidseptum cap.
 3. A method for making an acoustic structure according toclaim 1 wherein said resonator portion is perforated after said septumis inserted into said cell.
 4. A method for making an acoustic structureaccording to claim 1 wherein said flexible material is a plastic.
 5. Amethod form making an acoustic structure according to claim 1 whereinthe steps of forming said flexible material into said septum cap andlocating said septum cap into said cell are carried out together.
 6. Amethod for making an acoustic structure according to claim 1 whereinsaid cell in which said septum cap is located comprises at least onecorner and wherein some of said friction locking is provided by flexiblematerial that is compressed into said corner of the cell.
 7. A methodfor making an acoustic structure according to claim 1 wherein the widthof said anchoring surface and the bounce-back of said flexible materialis sufficient to provide substantially all of the friction locking ofsaid septum cap to said cell walls.
 8. A method for making an acousticstructure according to claim 1 wherein the combined surface areas ofsaid perforations in said resonator portion is below 20 percent of thesurface area of said resonator portion.
 9. A method for making anacoustic structure according to claim 1 wherein the size of saidperforations is from 0.002 inch to 0.015 inch.
 10. A method for makingan acoustic structure according to claim 1 wherein the thickness of saidflexible material is from 0.001 to 0.015 inch thick.
 11. A method formaking an acoustic structure, said method comprising the steps of: a)providing a precursor acoustic structure comprising: a honeycombcomprising a first edge and a second edge, said honeycomb furthercomprising a plurality of walls that extend between said first andsecond edges, said walls defining a plurality of cells wherein each ofsaid cells has a cross-sectional area measured perpendicular to saidwalls and a depth defined by the distance between said first and secondedges; a septum cap that is held in place within at least one of saidcells only by friction locking of said septum cap to said walls, saidseptum cap comprising a sheet of flexible material that has a thicknessand a perimeter, said sheet of flexible material being sufficientlyflexible to be folded and inserted into said cell to form said septumcap comprising a perforated resonator portion that extends in the sameplane transversely across said cell and which has an outer edge locatedat said walls and a flange portion that extends between the outer edgeof said perforated resonator portion and the perimeter of said sheet offlexible material, said flange portion extending parallel to said wallsand comprising an anchoring surface which is in frictional contact withsaid walls, said anchoring surface having a width wherein the width ofsaid anchoring surface and the bounce-back of said flexible material issufficient to provide friction locking of said septum cap to said cellwalls; and b) adhesively bonding said anchoring surface to the wall ofsaid honeycomb to form said acoustic structure.
 12. A method for makingan acoustic structure according to claim 11 wherein said sheet offlexible material is perforated prior to forming said sheet of flexiblematerial into said septum cap.
 13. A method for making an acousticstructure according to claim 11 wherein said resonator portion isperforated after said septum is inserted into said cell.
 14. A methodfor making an acoustic structure according to claim 11 wherein saidflexible material is a plastic.
 15. A method form making an acousticstructure according to claim 11 wherein the steps of forming saidflexible material into said septum cap and locating said septum cap intosaid cell are carried out together.
 16. A method for making an acousticstructure according to claim 11 wherein said cell in which said septumcap is located comprises at least one corner and wherein some of saidfriction locking is provided by flexible material that is compressedinto said corner of the cell.
 17. A method for making an acousticstructure according to claim 11 wherein the width of said anchoringsurface and the bounce-back of said flexible material is sufficient toprovide substantially all of the friction locking of said septum cap tosaid cell walls.
 18. A method for making an acoustic structure accordingto claim 11 wherein the combined surface areas of said perforations insaid resonator portion is below 20 percent of the surface area of saidresonator portion.
 19. A method for making an acoustic structureaccording to claim 11 wherein the size of said perforations is from0.002 inch to 0.015 inch.
 20. A method for making an acoustic structureaccording to claim 11 wherein the thickness of said flexible material isfrom 0.001 to 0.015 inch thick.